Box Tree Caterpillar Ranked As Top UK Garden Pest

The RHS has revealed the box tree caterpillar as the most enquired about garden pest on their telephone advice line last year.

This is little surprise as the spread of the caterpillar is growing at an alarmingly pace. At the moment gardens in London and the Home Counties are the worst affected. But there were been reports in 2017 of box tree caterpillars being found in Suffolk and Gloucestershire.

Dr Gerald Clover, head of plant health at the RHS, said people reporting sightings of box tree caterpillar increased from 536 in 2016 to 3,587 in 2017.

How to identify the box tree caterpillar

Box tree caterpillars are easily recognisable. If you do see one, act quickly as they can do a large amount of damage in a very short amount of time.

Box Tree Caterpillar Damage with Webbing and Frass

The first thing you may notice is defoliation. The is due to the caterpillar’s voracious appetite for box leaves. Looking closer you will see masses of sticky webbing across the box plants. This is the caterpillar’s defence mechanism. They surround themselves in webbing to deter predators. The webbing can be so extensive that it is difficult to penetrate with insecticides or similar sprays.

Box Tree Caterpillar

The box tree caterpillar is about 4cm long with black stripes and white dots on a light green body. The head of the caterpillar is shiny black and the body has small hairs. Another sign to look out for are green balls of ‘Frass’ (waste excreta) on the box leaves. Remove and humanely destroy the caterpillars as soon as you see them.

Box Tree Moth (Cydalima perspectalis)

The box tree moth starts laying eggs on the underside of box leaves in April. The young caterpillars are hard to see when they hatch as they are only 1-2mm long. But it is not long before the caterpillars grow to their full size and cause the most damage.

Box Tree Moth (Cydalima perspectalis) Pupa

If left alone the caterpillars will strip box plants of their leaves and attack the bark. Box will generally recover from leaf loss but bark damage can cause the plants to die.

The box tree moth can produce two or three generations of caterpillars in a year. They normally stop laying eggs in September but best to stay on alert for the box tree caterpillar until the end of October.